The present invention relates to selection of AC power phases and waveform control in order to select a waveform that optimizes delivered power within acceptable power and flicker limits.
A frequent issue in providing electric power to a device is control of the power provided to the device. Voltage can be decreased with simple resistors. Transformers can modulate between increased voltage and decreased current or vice versa. For devices that require varying amounts of power depending upon application, however, manipulation of the AC current waveform is a preferred method. Devices such as the heating lamps inside fuser rolls of electrostatographic printers are such an application. For fast warm-up cycles and during periods in which much heat is drawn from a fuser because of rapid printing speeds, full power drawn from an AC power source is desired. During non-imaging but machine-on periods, only power sufficient to maintain fuser temperature within a desired range is required. Little heat is drawn from the fuser during such periods. Diminished power can be accomplished by dropping out a major portion of the sinusoidal AC power signal in a manner to be described below. As printing frequency increases, more power needs to be applied in order to maintain the fuser at desired fusing temperatures. This increase in power can be accomplished by adding back portions of the AC power signal that were dropped during lower power periods.
Dropping out portions of the sinusoidal AC power signal is conventionally accomplished by phase control or cycle stealing methods. Conventional phase control acts over one half-cycle (180 degrees of the cycle) in a manner such as shown in FIG. 1. An essentially infinitesimal amount of a phase can be dropped, thereby enabling fine tuning of the amount of power reaching a device. Conventional cycle stealing enables the dropping of whole half cycles in a sequence of N half cycles as shown in FIG. 2. In conventional cycle stealing, each of the drops commences only at the zero crossing points, thereby offering only N different power levels. In order to avoid imparting a DC-offset, conventional cycle stealing utilizes an odd number of half-cycles and typically uses patterns of 3-half cycles, or 540 degrees.
Both conventional phase control and cycle stealing present problems depending upon particular applications. For conventional phase control, problems arise because the switching, or dropping of the signal, occurs every 180 degrees at points other than the zero crossing points. The result is harmonic current emissions in every half cycle (180 degrees). The harmonic current emissions are due to the non-zero crossing which induce secondary signals, or harmonics, sent back through the power lines. Where the power line is a public supply source such as a utility line, or mains, regulations place limits on the amount of harmonic emissions permitted by any device. With conventional phase control, these limits are easily exceeded. For conventional cycle stealing, several problems similarly result. First, as explained above, only N (typically 3 levels) of power are permissible, e.g. 0%, 33.3%, 66.67%, and 100%. Conventional cycle stealing is accordingly relatively inflexible and does not permit fine tuning of delivered power. Secondly, some modes of cycle stealing (particularly as N is made larger) cause voltage variation in the mains, which, in turn causes some light sources to appear to flicker as detected by the human eye. For instance, the second waveform shown in FIG. 2 drops 2 out of every 3 half-cycles, thereby yielding a 33% “on” signal. The two half-cycles in which the signal is “off” provides sufficient time for a typical lamp filament such as tungsten to cool significantly. The result is that when an “on” half cycle resumes, the filament has significantly less resistance, and an in-rush spike in current is created and a voltage fluctuation is sent down the mains. Such fluctuations in voltage affect light sources and other devices connected to the mains. The human eye is particularly sensitive to variations in light and particularly when the flicker is in a range nearing 8 Hertz. As a result of the above, human-detectable fluctuations of light caused by voltage artifacts introduced by devices is called “flicker” and is regulated in many jurisdictions.
A refinement to simple and conventional cycle stealing and phase control is pattern switching control methods exemplified by the sequences shown in FIG. 3. Such pattern switching control methods combine patterns of cycle stealing to obtain the relatively fine power control lacking with simple conventional cycle stealing. However, flicker problems persist. Each of the patterns labeled 2-5 in FIG. 4 contain two or more dropped half cycles. The result, as explained above, is a cooling of lamp filaments resulting in lowered resistance which, in turn, results in increases inrush current and flicker. The flicker introduced by some patterns is more noticeable than that introduced by other patterns because of the frequency of dropped double cycles. Pattern 4, which resembles simple cycle stealing, introduces the most noticeable flicker.
Another factor in designing controlled power circuits for devices is the power factor, defined as kW/kVA, where kW is the actual load power used by a device and kVA is the apparent load power as measured from the supply. The power factor of a signal is not particularly relevant for devices that require relatively little power. Where, however, devices require access to virtually all power that is available from the mains, the power factor becomes important. Available power is a function of current (I) multiplied by voltage (V). For simple AC current, the function is I multiplied by the root mean square voltage (RMS) as supplied. Current (I) is determined by the capacity of the supply. The maximum voltage (V) is also determined by the supply. As described above, however, methods of varying power by cycle stealing, phase control, pattern switching, chopping peak voltages, or similar methods all reduce power received by the device by dropping a portion of the AC signal. Since the full AC signal (kVA) is undiminished, such reduced actual load power (kW) used by the device reduces the power factor.
For devices which require high power factors, such as modern high speed printers, it is desired to provide a means for modulating the AC power signal to provide fine control of the power received by the device, minimize signal artifacts such as harmonics and flicker, while providing a relatively high power factor to ensure efficient use of the power supplied. Also, for energy conservation purposes, high power factors are desired.
One embodiment of the invention is an electrical signal carrying power for a device, comprising an alternating current waveform in which the waveform signal is dropped once at a non-zero crossing point in every 540 degrees.
Another embodiment of the invention is an electrical signal carrying power for a device, comprising an alternating current waveform in which the waveform signal is dropped once at any point in every 540 plus N times 180 degrees, where N is any non-negative number.
Another embodiment of the invention is a circuit for controlling the level of power delivered to a device, comprising: a zero crossing detector; a timing circuit; a combinatorial logic device that receives input from the zero crossing detector and the timing circuit; and a power switch for opening and closing the output signal in response to signals from the combinatorial logic device; wherein the output from the circuit is an alternating current waveform comprised of an alternating current waveform in which the waveform signal is dropped once at a non-zero crossing point in every 540 degrees.
Yet another embodiment of the invention is a method for controlling levels of alternating current electrical power delivered to a device, comprising: sensing, with a zero crossing detector, the time at which the alternating current signal comprises zero voltage; counting the alternating current half cycles; determining the phase angle that corresponds to the desired level of power within 540 plus N times 180 degrees, where N in any non-negative integer and where one phase angle corresponds to a non-zero crossing point; computing a time from a zero crossing point to the determined phase angle; and dropping the alternating current voltage at the determined phase angle once within the 540 plus N times 180 degrees.